1. Field of the Invention
The present invention relates to syringes and to a dose metering device for use in conjunction with a syringe and to syringes incorporating such devices.
2. Brief Description of the Background Art
Syringes of pocket pen-like shape and size are known.
GB-A-2109690 uses a mechanism operated by rotation of a cap mounted concentrically around the pen barrel. This rotational movement of the externally mounted cap is converted via a rotary ratchet and pawl mechanism and via a lead screw mechanism into axial movement of a rotating screw which drives a plunger down a cartridge in the barrel and expresses the dose. That arrangement permits the provision of a visual scale of micrometer type. The marker can be on the barrel or the cap and the scale on the cap or the barrel. It provides for a large range of doses and for fine gradation between doses. It also provides for the dose to be preset on the scale and then expression to be by a single movement up to a stop. This makes the injection stage independent of any need to assess what dose is being injected.
For some users the actual step of expression the dose causes anxiety and a device which automatically expressed the dose on demand would be attractive. Such a facility also could provide for a more repeatable and smooth injection of the fluid. This could reduce the shock of injection which can occur with some materials.